Fix deprecation warning on Faraday::Connection#authorization

Fixes #59

> WARNING: `Faraday::Connection#authorization` is deprecated; it will be removed in version 2.0.
> While initializing your connection, use `#request(:authorization, ...)` instead.
> See https://lostisland.github.io/faraday/middleware/authentication for more usage info.

Author: javierjulio

lib/dwolla_v2/token.rb

@@ -53,7 +53,7 @@ def inspect
 
     def conn
       @conn ||= Faraday.new do |f|
-        f.authorization :Bearer, access_token if access_token
+        f.request :authorization, 'Bearer', access_token if access_token
         f.headers[:accept] = "application/vnd.dwolla.v1.hal+json"
         f.request :multipart
         f.request :json